Prior art devices for determining the presence of carbon monoxide and other combustibles utilize techniques such as infrared adsorption, chemiluminescence, surface adsorption cells and the like. While such methods are capable of determining the required levels of carbon monoxide and other combustibles, they tend to be expensive, can be susceptible to long term drift, and can be difficult to calibrate.
One known type of combustibles sensor, as shown in U.S. Pat. No. 4,005,001, supplies a gaseous mixture of oxygen and fuel to first and second electrodes on opposite surfaces of an oxygen ion conductive solid electrolyte cell in the form of a disk having electrodes on opposite surfaces thereof, wherein the electrodes are composed of different materials each exhibiting a different catalytic action on the gaseous mixture at a given temperature. The difference in oxygen potentials established at the respective electrodes as a result of the dissimilar catalytic action produces oxygen ion conductivity in the solid electrolyte cell which produces an electrical signal having a magnitude which is indicative of the combustible present in the mixture. It has been found that with this disk type of electrolyte, concentration gradients with very low flow rates will force diffusion of the reacted and unreacted gas to opposite electrode surfaces which can cause confusing results.
Because of the foregoing, it has now become desirable to develop a combustibles sensor having improved flow sensitivity and reproducibility.